The answer is the Aar (another accepted spelling for the Aare.)
The Aare is a tribuatry. Another option is Ruhr as a 4-letter tributary of the Rhine.
It does not flow into a sea. It is a tributary of the Rhine River.
Amongst others - Madon, Terrouin, Moselotte, Dhron.
The Ruhr is a 217 Km long river in western Germany and is a tributary of the Rhine. It flows into the Rhine in Duisburg.
The river Rhine has approximately 40 different tributaries. These include the Main, Mosel, Neckar, and Meuse. the Mosel is the longest tributary.
Aar
The Arve River is a tributary of the Rhone River. It begins with the letter A.
c
The Isere River in France is a tributary of the Rhone River.
Aro
Isere
Oka